The present invention relates to overcurrent protection apparatus, in general, and more particularly to such apparatus under programmed control with selectable time-overcurrent operation to govern the interruption of current through at least one power line associated with either a ground phase or a power phase of a power system network.
Examples of types of overcurrent protection apparatus which operate with trip time delay characteristics in response to a sensed overcurrent condition in a power line include protective relays, automatic reclosures, circuit breakers and the like. Apparatus of this type generally protect electrical transmission and distribution systems and their connected loads from damage due to overcurrents by governing the interruption of current through their associated power lines in accordance with some time-to-trip vs. overcurrent coordinated relationship or predefined family of curves. Typically, the characteristics of these time-overcurrent curve families have been implemented using electro-mechanical, magnetic and thermal control circuits. Hence, once a particular time-overcurrent operation was characterized, it was intended to remain fixed and if a change in characterization was required usually some mechanical adaptation was needed like physically interchanging or adding special elements, for example.
Recently, electronic overcurrent protection control apparatus, and more particularly, electronic apparatus including programmed digital signal processors have been developed to provide greater functional flexibility in altering apparatus characterization with regard to time delayed current interruption control. Examples of programmed overcurrent protection control apparatus are found in the following U.S. Pat. Nos.:
4,446,498; "Electronic Control System For Overload Relay Or The Like"; issued May 1, 1984 to F. A. Stitch and assigned to Siemens-Allis, Inc.;
4,432,031; "Method For Overcurrent Protection"; issued Feb. 14, 1984 to W. J. Premerlani and assigned to General Electric Company;
4,377,837; "Circuit Interrupter With Over Temperature Trip Devise"; issued Mar. 22, 1983 to J. J. Matsko et al. and assigned to Westinghouse Electric Corp.;
4,377,836; "Circuit Interrupter With Solid State Digital Trip Unit And Positive Power-Up Feature"; issued Mar. 22, 1983 to R. T. Elms et al. and assigned to Westinghouse Electric Corp.;
4,371,947; "ROM Controlled Protective Relay System For Power Transmission Network"; issued Feb. 1, 1983 to Norio Fujisawa and assigned to Tokyo Chirbara Denki Kabushiki Kaisha;
4,339,802; "Digital Protective Relaying Device"; issued July 13, 1982 to M. Yamaura et al. and assigned to Tokyo Chirbara Denki Kabushiki Kaisha;
4,328,551; "Apparatus And Method For Protection Of Electric Power Transmission Lines And The Like"; issued May 4, 1982 to M. Yamaura et al. and assigned to Tokyo Chirbara Denki Kabushiki Kaisha;
4,272,816; "Overcurrent Protecting Apparatus"; issued June 9, 1981 to Y. Matsumato and assigned to Tokyo Chirbara Denki Kabushiki Kaisha.
While the aforementioned exemplary references are all directed to various novel methods and apparatus related to programmed digital signal processor control of overcurrent protection in power system networks, none of the above references appear to provide for both accurate characterization of the predefined time-to-trip vs. overcurrent curve families and the selectability of time overcurrent operation. The present invention is directed to programmed overcurrent protection control apparatus having aspects which are intended to fill these aforementioned gaps by providing accurate characterization of predefined families of time-overcurrent curves with a common programmed relationship based on settable parameters and the convenient setability of the parameters to select the curve family characterization suitable for the particular duty to which the overcurrent protection apparatus is being applied using the common programmed relationship.